1. Field of the Invention
The present invention generally relates to apparatus and method for producing a pattern using a stencil or mask and, more particularly to an apparatus and method for separating the mask or stencil from a surface on which the pattern has been formed.
2. Description of the Prior Art
Forming patterns of a material on a surface by extruding the material through a mask has been applied in many fields such as printing and engraving, graphic arts and manufacturing processes, particularly in the electronics industry. The ability to form a plurality of conductors simultaneously on a surface permits an inexpensive alternative wiring technique which also results in a compact wiring structure.
The scale of integration in integrated circuits has steadily increased over the years, resulting in integrated circuits of extreme complexity and capable of very comprehensive functions. It is often desirable in such devices to use a plurality of different integrated circuit chips within such a device to afford design flexibility, improved yield and to allow the use of different semiconductor circuit technologies within the same device. A particularly successful type of construction of such devices has been developed, which uses a plurality of layers of ceramic, glass or other insulative materials of relatively high thermal conductivity with conductors formed on the respective surfaces and in through-holes or vias thereof. This type of construction is generally referred to as a multi-layer ceramic (MLC) structure. Since circuits constructed in this way are three-dimensional, a high degree of complexity is possible.
In such structures, the conductors are usually formed by a pattern of conductive paste. The conductive paste pattern is usually formed by extruding the paste, which is usually highly viscous, through a stencil or mask by passing an extrusion nozzle over the mask, located on the ceramic layer or "green sheet". The mask will typically be formed from a metal sheet having an apertured pattern. The mask structure may also have downward opening cavities supported periodically by mask stand-offs for forming large pattern areas. Such stand-offs are necessary to avoid flexing of the mask (e.g. being pushed away from the nozzle or being otherwise deformed) causing displacement of the conductive paste and shorts or voids in the conductive pattern. The term "stand-offs" will be used hereinafter to refer to a mask portion at a location where paste is not to be applied and where the mask rests directly on the surface on which the pattern is to be formed. It is also common in design rules for the fabrication of such masks to specify a characteristic distance for stand-off spacing, based on maximum uninterrupted feature size to be produced and the locations of stand-offs may be thought of as a regularly spaced array with the spacing corresponding to the characteristic distance.
After extrusion is complete, the mask must be separated from the green sheet without disturbing the paste pattern. Experience with manufacture of MLC devices has indicated that, at present, most defects are caused during separation of the mask from the green sheet. It should be noted that these steps are common to all screening processes for forming patterns on a surface and, while the invention will be disclosed in relation to a MLC manufacturing process in which pattern accuracy is very critical, applicability is not to be considered limited thereto.
As disclosed in U.S. Pat. No. 4,902,371, to Andris et al., assigned to the assignee of the present invention and hereby fully incorporated by reference herein, most pattern defects occur near the center of the mask. Since the mask and the green sheet are both supported at their edges, as the two parts are separated, both assume a slightly bowed shape. Near the edges, the mask apertures place a shear force on the viscous paste and the mask usually separates cleanly from the green sheet surface. However, near the center, the mask places tension on the paste and can pull portions of paste off the green sheet under certain circumstances, causing pattern voids.
More often, as the mask separates from the green sheet with a snapping action at the center of the mask, spikes of paste are formed at the edges of the paste pattern in the central area. Since the spacing of areas of the conductive paste pattern is often very fine, these spikes often cause shorts between conductors either by simply collapsing onto the green sheet or by having the excess paste therein spread out during further processing such as when the green sheets are laminated into the final MLC device.
Also, it is at this point in the screening process that greatest mechanical and acceleration forces are exerted on concentrated areas of the mask, causing deflection and distortion thereof, particularly in areas where mask features are very small. Such distortions will eventually cause metal fatigue and fracture of smaller features of the metal mask. Further, screened material adhering to the mask during this stage may cause defects in subsequently masked green sheets.
It should be noted that even if it is attempted to remove the mask by peeling it away from the surface from one end, the limitations on the flexure of the mask and the necessity that the mask not be shifted relative to the green sheet make the application of tension to the paste in some area unavoidable. Therefore, it is to be understood that the term "central area" refers to the area where the mask and surface finally separate and in which tension of the screened material is greatest, although the area may not be centrally located on the mask or surface and may, in fact, may be moved from the center by design or simply variation in the structures used to support and manipulate the mask and surface. However, for purposes of visualizing the circumstances of the invention in the environment of an MLC screening operation, the central area is about a 2.25" diameter generally circular area and corresponds to about 10% to 25% of the mask area. However, for purposes of counting defects in the following discussion, the central area is considered to be a 2.25".times.2.25" square area.
As disclosed in the above-incorporated patent, much of the spiking in the central area of the patterned surface is due to the relatively uncontrolled snapping action of the mask and green sheet at the time of separation. In order to achieve control of mask motion, a shock absorber including a shock absorber pad was lowered onto the central area of the mask and a force applied thereto to balance the forces due to the adhesion to the surface. In this way, the central area of the mask could be removed from the surface in a smooth and controlled motion. However, the speed of separation of the mask from the surface remains critical to the reduction of spiking during this process.
In an effort to reduce the spiking of the conductive paste, mask separation speed has been varied and it has been found that a reduced separation speed reduces the defects which occur in the central area of the mask. However, this possibility carries a high cost in terms of the throughput of the system. For example, a ten second separation time has yielded defects in the central area and, to a lesser degree, throughout the patterned area even when a clean mask was used for each screening. A fifteen second separation time yielded only about half as many defects as the ten second separation time in both the central area and in other areas of the green sheet. If a second screening was attempted without cleaning the mask between screenings, defects in the central area and peripheral area of the mask were approximately tripled. To increase throughput, multiple screenings for each cleaning of the mask is very desirable but virtually precluded at ten and fifteen second separation speeds by the increased incidence of defects.
Although no defects were noted outside the central area when a thirty second separation time was used even using the mask for a second screening process before cleaning, the number of central area defects, while lowered, still remained significant and comparable to a ten second separation time after screening with a clean mask.
In order to increase throughput of the system, alteration of speed of separation by reducing separation speed when the central area was reached has been attempted. However, without actual monitoring of the separation process, the point at which speed should be reduced could only be estimated. It was found that the geometry of the mechanical system during separation would change significantly with very slight changes in paste viscosity and, more importantly, randomly from green sheet to green sheet due to slight changes in the amount of elastic deformation of the green sheet which occurred in response to the separation force applied to it through the paste.
For the same reason, attempts to control speed changes of the process through a feedback system have not been successful because of this variability of the geometry of the parts during the process. In essence, it was not possible to accurately sense the separation because of variability of the location of the mask and surface at a particular point in the separation process. Additionally, it was very difficult to provide space at an appropriate location, particularly below the mask, to even attempt such automatic control without causing interference with the process. It should be noted that even if appropriate space could be provided at such a location, any sensor and optical sensors, in particular, would be subject to contamination from the screened material which would degrade reliability and system performance. Further, such speed alteration would cause a 2% to 5% reduction in production capacity while there would be no assurance that the process would be performed optimally.
It should also be noted that in using the apparatus disclosed in the above incorporated patent, a shock absorber is brought into contact with the mask during separation. During the separation period, the relative motion of parts is very slow to reduce defects as noted above. After separation is complete, the parts can be moved at high speed to a home position to allow another screening operation to be performed. However, the variability of the separation operation, regardless of the speed at which it is done, prevents the high speed movement to the home position from being initiated immediately after separation is complete. For the same reasons pointed out above with regard to the difficulty of monitoring the stages of the separation process, extra time must be provided prior to initiating high speed movement to ensure that the mask and surface have, in fact separated. During this additional time, the shock absorber remains in contact with the mask and the mask and surface move apart slowly. Since the mask may be damaged and pattern defects are virtually assured if high speed movement is initiated before separation is complete, the amount of additional time is usually on the order of 3-5 seconds to cover virtually all of the statistical variation in actual separation times. This amount of time corresponds to a substantial expense in a production environment and represents about a 10% reduction in throughput of the screening operation compared to the throughput otherwise possible.